To gain a better understanding of the role of the kallikrein-kinin system in the kidney, we have studied the direct effect of bradykinin on sodium and potassium transport on the isolated perfused rat cortical collecting duct. Sodium, potassium and insulin concentrations were measured in the perfusion fluid, bathing fluid, and in the collected fluid to determine the fluxes of each substance across the tubule epithelium. The potential difference across the epithelium was also measured. Arginine vasopressin and/or bradykinin were placed in the bath or in the perfusate. Animals were also pretreated with deoxycorticosterone. Tubules were perfused at the rate of 1.5-2.5 nL/min/mm. Deoxycorticosterone pretreatment of rats for 7-12 days caused a marked increase in sodium absorption and potassium secretion and changed the transepithelial P.D. to lumen negative. There was no fluid transport. Addition of bradykinin to the bath significantly decreased sodium reabsorption without affecting potassium secretion. However, bradykinin did not significantly change the P.D nor was there any demonstrable fluid transport. Compared with bradykinin, vasopressin caused a striking increase in sodium absorption in association with a significant fluid absorption and marked increase in the lumen-negative P.D. In addition, vasopressin significantly increased potassium secretion. All effects of vasopressin were reversed when the hormone was removed from the bath. Of greater interest, when bradykinin was added to the bath already containing vasopressin, there was a significant reversible fall in sodium reabsorption and fluid transport with no change in potassium secretion or P.D., effects previously observed with bradykinin in the absence of vasopressin (albeit to a lesser extent due to the lower baseline values in the absence of vasopressin). Addition of bradykinin to the perfusion fluid up to 10-6 M was without effect. The combined data show, for the first time, a direct inhibitory effect of bradykinin on sodium reabsorption. In a separate study, kininogen was found in washed homogenates of rabbit kidney.